Alternator jump charging system

ABSTRACT

A system for charging a first source from a second source includes an alternating current (ac) voltage source having an internal inductance, a switching power converter coupled to the ac voltage source and the first source, a control circuit coupled to the switching power converter, a connecting system for selectively connecting a positive terminal of the second source to the ac voltage source and a jump charging controller coupled to the control circuit and operative to enable the switching power converter to be used in conjunction with the ac voltage source inductances as a dc/dc converter to charge the first source from the second source.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) from U.S.provisional application No. 60/177,752 filed on Jan. 24, 2000 and U.S.provisional application No. 60/184,006 filed on Feb. 22, 2000.

FIELD OF THE INVENTION

This invention relates generally to alternator systems and moreparticularly to alternator systems used in vehicles.

BACKGROUND OF THE INVENTION

As is known in the art, an alternator is an alternating current (ac)output generator. To convert the ac voltage to direct current (dc) foruse in charging batteries or supplying dc loads, for example, arectifier system is used. Sometimes, the alternator is referred to as anac machine or more simply a machine and the combined generator/rectifiersystem is referred to as an alternator or an alternator system.

In many cases (including automotive alternators), a diode rectifier isused to rectify the ac voltage produced by the generator. The acgenerator can be modeled as a three-phase voltage source and a set ofinductors.

In a so-called wound-field machine, the output voltage or current can becontrolled by varying the current in a field winding which in turnvaries the ac voltage magnitudes. The advantage to this approach is theextreme simplicity and low cost of the system. One particular type ofwound field machine is a so-called wound-field Lundell-type alternator.A lundell machine is characterized by the way the rotor/field of themachine is constructed, the details of which are well-known to those ofordinary skill in the art. Significantly, the construction techniquesused to manufacture Lundell-type result in an ac machine which isrelatively inexpensive but which has a relatively high leakageinductance or reactance. Wound-field Lundell-type alternators are almostuniversally used in the automotive industry primarily because they arereliable and inexpensive. One problem with wound-field Lundell-typealternators, however, is that the relatively high machine inductancestrongly affects the machine performance. In particular, due to the highinductance of the Lundell machine, it exhibits heavy load regulationwhen used with a diode rectifier. That is, there are significant voltagedrops across the machine inductances when current is drawn from themachine, and these drops increase with increasing output current andmachine operating speed. Consequently, to deliver substantial currentinto a low dc output voltage, the ac machine voltage magnitudes have tobe much larger than the dc output voltage.

For example, in a typical high-inductance automotive alternatoroperating at relatively high speed, the internal machine voltagemagnitudes are in excess of 80 V to deliver substantial current into a14 V dc output. This is in contrast with a low-reactance machine with adiode rectifier, in which the dc output voltage is only slightly smallerthan the ac voltage magnitudes.

In order to control output voltage or current, a controlled rectifier issometimes used instead of field control. One simple and often-usedapproach for controlled rectification is to replace the diodes of adiode rectifier with thyristor devices. For example, as described in J.Schaefer, Rectifier Circuits, Theory and Design, New York: Wiley, 1965and J. G. Kassakian, M. F. Schlecht, and G. C. Verghese, Principles ofPower Electronics, New York: Addison-Wesley, 1991 thyristor devices canbe used in a semi-bridge converter. With this technique, phase control(i.e. the timing of thyristor turn on with respect to the ac voltagewaveform) is used to regulate the output voltage or current. One problemwith this approach, however, is that it can be relatively complex from acontrol point of view. This is especially true when the alternator mustprovide a constant-voltage output.

Alternatively, rather then using phase control, control is sometimesachieved using switched-mode rectification. With the switched-moderectification technique, fully-controllable switches are used in a pulsewidth modulation (PWM) fashion to produce a controlled dc output voltagefrom the ac input voltage. This approach, which typically utilizes afull-bridge converter circuit, often yields high performance at theexpense of many fully-controlled PWM switches and complex controlcircuits and techniques.

One relatively simple switched-mode rectifier that has been employed foralternators attached to wind turbines is described in an articleentitled “Variable Speed Operation of Permanent Magnet Alternator WindTurbines Using a Single Switch Power Converter,” by G. Venkataramanan,B. Milkovska, V. Gerez, and H. Nehrir, Journal of Solar EnergyEngineering—Transactions of the ASME, Vol. 118, No. 4, November 1996,pp. 235-238. In this approach, the alternator includes a rectifiercomprising a diode bridge followed by a “boost switch set” provided froma controlled switch (such as a MOSFET) and a diode. The switch is turnedon and off at a relatively high frequency in a PWM fashion. Thisapproach is utilized along with PWM switching generated by acurrent-control loop to simultaneously control the output current andturbine tip speed of a permanent magnet alternator. The approach isspecifically applied to a low-reactance (i.e. low-inductance)permanent-magnet ac machine where the battery voltage is higher than theac voltage waveform. It should be noted that the rectifier system istopologically the same as the Discontinuous Conduction Mode (DCM)rectifier described in an article entitled “An Active Power FactorCorrection Technique for Three-Phase Diode Rectifiers,” by A. R. Prasad,P. D. Ziogas, and S. Manias, the IEEE Trans. Power Electronics, Vol. 6,No. 1, January 1991, pp. 83-92, but the operating mode and controlcharacteristics of the single switch power converter and DCM rectifierare very different.

Another controlled rectifier approach for alternators is described inU.S. Pat. No. 5,793,625, issued Aug. 11, 1998. This patent describes acircuit which utilizes the application of boost mode regulatortechniques to regulate the output of an ac source.

The source inductance becomes part of the boost mode circuit, thusavoiding the losses associated with the addition of external inductors.When a three-phase alternator is the power source, the circuit comprisesa six diode, three-phase rectifier bridge, three field effecttransistors (FETs) and a decoupling capacitor. The three FETs provide ashort circuit impedance across the output of the power source to allowstorage of energy within the source inductance. During this time, thedecoupling capacitor supports the load. When the short circuit isremoved, the energy stored in the inductances is delivered to the load.Because the circuit uses the integral magnetics of the ac source toprovide the step-up function, a relatively efficient circuit isprovided. The duty cycle of the switches (operated together) is used toregulate the alternator output voltage or current. The rectifier canthus be used to regulate the output voltage and improve the currentwaveforms for low-reactance machines that would otherwise operate withdiscontinuous phase currents.

While regulating output voltage or current with a boost circuit of thistype may be useful in permanent magnet alternators having relatively lowinductance characteristics, this method is not useful with alternatorshaving a relatively large inductance characteristic and a wide operatingspeed range such as in wound-field Lundell-type alternators forautomotive applications.

To understand this, consider that in a system which includes analternator coupled to a boost rectifier, the output voltage is fullycontrollable by the boost rectifier when the internal machine voltagesare the same magnitude or lower than the dc output voltage as described,for example, in the above referenced Venkataramanan paper. However, ifthe internal machine voltages become significantly larger than thedesired dc output voltage, then the output voltage cannot be regulatedby the boost rectifier independent of load without inducing unacceptablyhigh currents in the machine. The typical automotive Lundell alternatorpresents this problem.

At the present, high-reactance Lundell-type alternators with dioderectifiers and field control are widely used in the automotive industry.Moreover, there is a very large infrastructure dedicated to themanufacture of Lundell-type alternators. However, design of thesealternators is becoming increasingly more difficult due to continuallyrising power levels required in vehicles and in particular required inautomobiles.

As is also known, the average electrical load in automobiles has beencontinuously increasing for many years. The increase in electrical loadis due to the demand to provide automobiles and other vehicles withincreasingly more electronics and power consuming devices such asmicroprocessors, electric windows and locks, electromechanical valves,and electrical outlets for cell phones, laptop computers and otherdevices. Such additional electronics results in a need for moreelectrical energy in automobiles and other vehicles.

Because of this increase in electrical load, higher power demands arebeing placed on automotive alternator systems. Furthermore, theincreasing power levels have motivated the adoption of a new higherdistribution voltage in automobiles to augment and/or replace thecurrent 14 V distribution system. In some cases, a single high-voltageelectrical system may likely be used (e.g. a 42 volt electrical system).In other cases, a dual-voltage electrical system may be used whichincludes a first relatively high-voltage system (e.g. a 42 V electricalsystem) and a second relatively low-voltage system (e.g. a 14 Velectrical system.) The high-voltage electrical system will be used topower vehicle components which require a relatively large amount ofpower such as a starter motor of a vehicle. When retained (in thedual-voltage case), the low-voltage system will be used to power vehiclecomponents that benefit from a low-voltage supply such as incandescentlamps and signal-level electronics.

A dual- or high-voltage system having a starter motor coupled to ahigh-voltage bus requires a charged high-voltage battery to start. Incases where the high-voltage battery is not fully charged or isdepleted, it would be desirable to be able to charge the depletedhigh-voltage battery from a low-voltage source in order to provide“jump-start” capability for dual/high voltage systems. In an automobilewhich includes only a single high-voltage system, one may desire totransfer energy from a low-voltage power source, battery or alternatorof a different vehicle to the high-voltage system. In a dual-voltagesystem, one may desire to transfer energy from a low-voltage battery ofthe dual-voltage system to the high voltage battery of the dual-voltagesystem or from the low-voltage battery or alternator of a differentvehicle or other low-voltage source to the high voltage battery of thedual-voltage system.

It would, therefore, be desirable to provide a means by which the poweroutput capability of an alternator can be increased. It would also bedesirable to provide an alternator which is capable of efficientoperation at a plurality of different voltage levels.

It would be further desirable to provide an alternator which is capableof operating in a dual voltage automobile. It would be still furtherdesirable to provide a system and technique for transferring energy froma low-voltage battery of a first vehicle or from an alternator of asecond different vehicle or other source to a high-voltage bus of thefirst vehicle.

SUMMARY OF THE INVENTION

In accordance with the present invention, a system for charging abattery at the output of the switched-mode rectifier includes means forselectively connecting the positive terminal of a charging source (whichmay be a low-voltage source) to the machine neutral point. This may bedone via a connector or a switch (such as a mechanical switch, relay, orsemiconductor switch), or by another similar means. The negativeterminal of the charging source is connected to system ground (as is thenegative terminal of the high-voltage battery). In this configurationthe ac machine inductances in conjunction with the switched-moderectifier can be used as a dc/dc converter to charge the high-voltagebattery from the charging source. In one embodiment, the switched moderectifier includes a plurality of metal oxide semiconductor field effecttransistors (MOSFETs). When the MOSFETs are turned on, the current inthe ac machine inductances increases, drawing energy from thelow-voltage charging source and storing it in the machine inductances.When the MOSFETs are turned off, some of this energy plus additionalenergy from the charging source are transferred to the high-voltagebattery through the diodes. The high-voltage battery may be charged froma low-voltage charging source (for jump-starting purposes, for example)using this method.

It should be recognized that this approach may be utilized indual-voltage systems as well. In the case of a dual-voltage system, thecharging source may be the low-voltage battery of the same vehicle, orit may be supplied from a different vehicle or source. Again, a means isprovided for selectively connecting a neutral of the ac machine to thedesired charging source. In a dual-voltage system charging from its ownlow-voltage battery, this connection may be conveniently provided by arelay connecting the machine neutral to the positive terminal of thelow-voltage battery, for example.

In accordance with a further aspect of the present invention, a methodfor charging a battery at an output of a switched-mode rectifierincludes the steps of connecting the positive terminal of a chargingsource to a neutral point of an ac machine, connecting the negativeterminal of the charging source to system ground (it is assumed that thenegative terminal of the high voltage source is already connected toground), increasing the current in the ac machine to draw energy fromthe charging source and storing the energy in the machine inductancesand transferring the energy plus additional energy from the chargingsource to the high-voltage battery. With this method, the high-voltagebattery may be charged from a low-voltage source for jump-startingpurposes, for example.

The connection between the positive terminal of the low voltage sourceand the ac machine neutral point may be provided via a connector or aswitch (such as a mechanical switch, relay, or semiconductor switch), orby another similar means. The current in the ac machine may be increasedto draw energy from the low-voltage source by providing a low impedancepath between the output terminals of the alternator and system ground.This may be accomplished, for example, by turning on switches in aswitched mode rectifier coupled to the ac machine.

In this configuration, the ac machine inductances in conjunction withthe switched-mode rectifier can be used as a dc/dc converter to chargethe high-voltage battery from the charging source. When the switches ofthe switched-mode rectifier, which may be provided as MOSFETs forexample, are turned on, the current in the machine inductancesincreases, drawing energy from the low-voltage source and storing it inthe machine inductances. When the switches are turned off, some of thisenergy plus additional energy from the charging source are transferredto the high-voltage battery through the diodes.

It should be recognized that this approach may be utilized indual-voltage systems as well. In the case of a dual-voltage system, thecharging source may be the low-voltage battery of the same vehicle, orit may be supplied from a different vehicle or source. Again, a means isprovided for selectively connecting the alternator machine neutral tothe desired charging source. In a dual-voltage system charging from itsown low-voltage battery, this connection may be conveniently provided bya relay connecting the machine neutral to the positive terminal of thelow-voltage battery, for example.

It should also be recognized that this approach may be used in systemsemploying rectifier types other than a boost rectifier, such as thosedescribed below. With other rectifier types, the charging source voltagemay in fact be the same or larger in magnitude than the high-voltagebattery at the switched-mode rectifier output. It should also berecognized that connection points other than the machine neutral may beused with this approach. Again, in these cases, the alternatorinductances and switched-mode rectifier may be used as a dc/dc converterto provide controlled charging from the charging source to thehigh-voltage battery.

In accordance with a still further aspect of the present invention, analternator system having an alternating current (ac) voltage sourcecontrolled by a field current regulator and an internal inductanceincludes a switched-mode rectifier coupled to the ac voltage source, aswitched-mode rectifier (SMR) control circuit coupled to theswitched-mode rectifier and a speed sensor coupled to the SMR controlcircuit.

With this particular arrangement, an alternator system capable ofincreased alternator system power output is provided. The speed sensorsenses a frequency or operating speed of the ac voltage source andprovides a signal representative of the frequency or speed to the SMRcontrol circuit. In response to the frequency or speed informationprovided thereto, the SMR control circuit provides a duty ratio signalto the switched-mode rectifier which causes the switched-mode rectifierto operate with a particular duty cycle. The switched-mode rectifierduty cycle is thus selected based upon the frequency or speed of the acvoltage source. When the switched-mode rectifier is operated in thismanner, the alternator system achieves levels of power and performancethat are higher than those conventionally achieved.

The switched-mode rectifier operates at a duty cycle selected to providea controlled transformation of voltage and current between terminals ofthe ac voltage source and output terminals of the alternator system andconverts an ac voltage from the ac voltage source to a direct current(DC) voltage. In this manner the switched-mode rectifier transforms thevoltage at the output of the ac voltage source such that theswitched-mode rectifier stage extracts relatively high levels of powerand performance from the ac voltage source.

Because of the load regulation in a high reactance machine, for maximumfield current, the output power that the ac machine delivers is afunction of the speed of the machine and the effective voltage seen bythe ac machine. For a given speed, there is a single effectivealternator voltage at which maximum power output will be achieved (asillustrated in the curves of FIG. 3 discussed below). The switched-moderectifier provides a controlled transformation of voltage and currentbetween the terminals of the alternator machine and the alternatorsystem output. The transformation is controlled by a duty ratio d. Inthe case where the switched-mode controller includes a boost rectifiercircuit, the machine sees a local equivalent voltage v_(x) that is (1−d)times the output voltage, and the output receives a current that is(1−d) times the machine current, so that the boost rectifier appears asan ideal transformer with turns ratio 1−d. By controlling the duty ratioof the switched-mode rectifier as a function of speed (rpm) one canensure that the alternator machine can always achieve its maximum poweroutput independent of the fixed alternator system output voltage. Itshould be appreciated that the speed sensor can sense any parameter orcombination of parameters related to ac machine speed (e.g. enginespeed, frequency, alternator speed, frequency, alternator back emf,etc.) and provide an appropriate signal to the SMR control circuit.

At the same time the present invention provides a circuit that isrelatively simple and inexpensive. The alternator system can alsoinclude a field controller and a field current regulator coupled to theac voltage source that controls the ac voltage source magnitude. Thefield control can be used as a primary means for regulating outputvoltage or current in the wound-field alternator. The switched-moderectifier stage is controlled and acts as a second control handle toextract relatively high levels of power and performance from thealternator.

In one embodiment the alternator system can optionally control theswitched-mode rectifier duty ratio as a function of both the alternatorspeed and the field current magnitude. To achieve maximum power from themachine (at full field current) it is sufficient to control the dutyratio as a function of speed. By controlling the duty ratio as afunction of both speed and field current, it is possible to achieveimproved operation (e.g. higher efficiency) at partial load in additionto the improvement in maximum output power. It should be appreciatedthat the field current can be determined by any parameter or combinationof parameters related to field current, e.g. field current, averagefield voltage, field controller duty ratio, alternator back emf, fieldwinding magnetic field strength, etc. It should also be appreciated thatthe switched-mode rectifier duty ratio can be controlled based onmeasurements related to joint functions of field current and speed, suchas alternator back emf (which is related to the product of alternatorspeed and field current).

In a different embodiment, the alternator system can optionally controlthe switched mode rectifier duty ratio as a f-unction of the machineback emf, which is related to the product of alternator speed and fieldcurrent. Controlling the switched-mode rectifier as a function ofmachine back emf will ensure that the alternator machine can alwaysachieve its maximum power output independent of the fixed alternatorsystem output voltage, and will also provide high efficiency operationat output power levels below maximum. A machine back emf sensor (whichmay comprise a sense winding and processing electronics or other meansof measuring or estimating the back emf) may be provided in place of orin addition to the speed sensor in this embodiment.

In one embodiment, the alternator system can optionally include a faultprotection controller coupled to the SMR control circuit. The faultprotection controller operates under fault conditions (e.g. load dump),and overrides the other controllers in the alternator system based onoutput voltage when a load dump occurs.

In operation, in response to detection of a fault condition by the faultprotection controller, (e.g. when a significant over-voltage is detectedat the output terminals of the alternator system) the fault protectioncontroller overrides both the field and switched-mode rectifiercontrollers such that the field current is driven down and theswitched-mode rectifier limits the load dump transient at the output. Inthis manner, the fault protection controller provides a means forimplementing load dump protection. Thus, inclusion of the faultprotection controller provides an alternator system having a greaterdegree of circuit protection than can be achieved with a conventionaldiode rectifier.

In one embodiment the alternator system can optionally include aconnecting system for selectively connecting the positive terminal of acharging source to the machine neutral point, along with a jump chargingcontroller coupled to the SMR control circuit. This enables theswitched-mode rectifier to be used in conjunction with the alternatormachine inductances as a dc/dc converter to charge the battery at theoutput of the switched-mode rectifier from the charging source. Theinclusion of these elements provides an important improvement in thealternator system functionality over what is achieved in conventionalsystems.

The circuit of the present invention is well suited to use withhigh-reactance wound-field alternators, including automotiveLundell-type alternators and therefore finds immediate applicability inuse with automotive alternators. The present invention also finds use inany application which requires an alternator including but not limitedto the petroleum exploration industry, where a downhole alternator,connected to a turbine driven by drilling mud, is used as a downholepower source in directional drilling operations. The invention alsofinds use in generators for marine and aerospace applications, portablegenerators and backup power supplies.

With the present invention, relatively high power levels can be achievedwithin the existing manufacturing framework and with existing machinesizes at relatively low cost. Furthermore, the so-called load dumpproblem associated with Lundell and other wound field types ofalternators is overcome by the addition of some control circuitry (e.g.a fault protection controller coupled to sense voltage levels at theoutput or at other locations of the alternator system), a relativelysmall change in the rectifier stage (e.g. coupling of the rectifierstage to the fault protection controller) and minor adjustments in themachine design so that the peak of the machine's output power versusoutput voltage curve for constant speed with diode rectification matchesthe desired output voltage at a desired cruising speed, rather than atidle.

Changes to the machine, for example, could be implemented by providingthe ac machine having a particular number of turns and a particular wiregauge selected such that at a desired cruising speed the peak of the acmachine's output power vs. dc output voltage curve for dioderectification occurs at the desired output voltage. Those of ordinaryskill in the art will appreciate of course that other parameters of theac machine in addition to or in place of the number of turns and wiregauge may also be appropriately selected to achieve desired operation ofthe ac machine. Thus, relatively simple modifications to the winding ofa conventional alternator can be made for good operation in accordancewith the present invention at any desired output voltage.

The steps to wind of the alternator for a desired voltage are asfollows: first, select a suitable cruising speed to design for andsecond, choose the number of alternator stator winding turns such thatthe peak of the output power versus output voltage curve (for dioderectification) at the design speed reaches its maximum at the desiredoutput voltage. It should be noted that there are other moresophisticated modifications that can be made to an alternator for goodoperation in accordance with the present invention, such as reoptimizingthe magnetic and thermal design of the alternator.

In addition to trends towards higher power in automotive alternators,there will be a need for high-power automotive alternators whichgenerate power at higher voltages (e.g. at a voltage of 42 volts (V)instead of 14 V). With the present invention, by changing only therectifier stage and control circuits even present 14 V ac machinedesigns are suitable for high-power operation at 42 V output. That is,by merely replacing the diode rectifier stage of a conventionalalternator system with the switched-mode rectifier of the presentinvention, changing the field controller to one that takes input from afault or load-dump protection controller, and replacing the controlcircuitry with appropriate new controls as described above, presentmachines designed for operation at 14 V can operate at 42 V. It shouldbe appreciated that with such modifications operation can be achieved atother voltages including but not limited to 42 V.

With such a relatively simple change, it may be possible to manufactureboth 14 V and 42 V versions of an alternator on the same manufacturingline. Thus, the new invention is timely for meeting the demands ofhigher power and higher voltage alternators in the automotive industrywhile remaining within the existing manufacturing framework andovercoming the present day load dump transient problem.

It should be noted that in prior art systems employing a switched-moderectifier, the rectifier is used to regulate the output voltage. This isin contrast to the operation of an alternator system which operates inaccordance with the present invention in which field control is used toregulate the output voltage and the switched-mode rectifier functions toprovide “load matching” between the machine and the load so that muchhigher levels of power can be extracted from the alternator than couldbe achieved with a diode rectifier.

The differences between such prior art systems and the system of thepresent invention become clear when one considers the details of thecontrol circuitry. In a prior-art system using a switched-moderectifier, the alternator system output voltage (or current) is an inputto the controller of the switched-mode rectifier. In accordance with thepresent invention, however the frequency or speed of the ac machine isprovided to the controller of the switched-mode rectifier. Thus, the SMRneed only utilize the frequency or speed of the ac machine to determinethe duty ratio of the switched-mode controller. A field controller and afield current regulator or other output voltage control means coupled tothe ac machine may be used to primarily regulate the output voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this invention, as well as the inventionitself, may be more fully understood from the following description ofthe drawings in which:

FIG. 1 is a block diagram of an alternator system;

FIG. 1A is a block diagram of an alternator system;

FIG. 1B is a block diagram of an alternator system;

FIG. 1C is a block diagram of an alternator system;

FIG. 1D is a block diagram of an alternator system;

FIG. 2 is a schematic diagram of an alternator system;

FIGS. 2A and 2B show several waveforms at various points in analternator system of the type described in conjunction with FIG. 2;

FIG. 2C is a plot of duty ratio vs. alternator speed;

FIG. 2D is a schematic diagram of an alternator system which utilizesboth field control and a speed sensor;

FIG. 2E is a plot of duty ratio vs. alternator speed;

FIG. 2F is a schematic diagram of an alternator system which utilizes aback emf sensor;

FIG. 2G is a schematic diagram of an alternator system which utilizescompensator and limiter circuits;

FIG. 3 is a plot of alternator output power versus alternator outputvoltage at full field;

FIG. 3A is a plot of alternator output power versus alternator outputvoltage;

FIG. 4 is a schematic diagram of an alternator system;

FIG. 5 is flow diagram showing the steps to design an alternator inaccordance with the present invention;

FIGS. 6-10 are series of schematic diagrams illustrating different typesof switched-mode rectifier circuit topologies which can be used inaccordance with the present invention;

FIG. 11 is a schematic diagram of a prior art dual-rectified alternatorsystem;

FIG. 12 is a schematic diagram of a dual-rectified alternator systemwhich operates in accordance with the principles of the presentinvention;

FIG. 12A is a schematic diagram of a dual-rectified alternator systemhaving a control system which receives a pair of input signals;

FIG. 13 is a block diagram of a dual-output alternator system using adual wound alternator machine; and

FIG. 14 is a is a schematic diagram of an alternator system including ameans for selectively connecting a low-voltage source to a neutral of anac machine; and

FIG. 15 is a schematic diagram of a dual winding alternator systemincluding a means for selectively connecting a low-voltage source to aneutral of an ac machine.

DETAILED DESCRIPTION OF THE INVENTION

As noted above, those of ordinary skill in the art sometimes refer tothe ac machine itself as an alternator while at other times those ofordinary skill in the art refer to the combination of the ac machinecoupled to a rectifier circuit also as an alternator. To promote clarityin the text, the term “alternator system” will be used herein todescribe a system which includes an ac generator portion and a rectifierportion. The ac generator portion may also be referred to as an “acmachine,” an “ac generator,” a “generator” or an “alternator” while therectifier portion of an alternator system will be referred to herein asa “rectifier” or a “rectifier circuit.” The term “ac voltage source” isintended to cover any type of source which can be used with the presentinvention including but not limited to an alternator.

In the description hereinbelow, reference is sometimes made to an acmachine having a particular number of phases. Those of ordinary skill inthe art will appreciate, of course, that the concepts described hereinapply equally well to ac machines having any number of phases includingsingle phase or any poly-phase ac machines. Reference is also sometimesmade herein to switched-mode rectifiers and SMR controllers having aparticular topology. Those of ordinary skill in the art will appreciatethat the principles of the present invention can be implemented using avariety of switched-mode rectifier topologies and that those presentedherein are only examples and should not be construed as limiting. Itshould be appreciated that any switched-mode rectifier topology capableof implementing the desired control function can be used.

Reference is also sometimes made herein to alternators or alternatorsystems operating at a particular voltage level or within a range ofvoltage levels such as 14 volts (V) or 42 V. it should be understoodthat the principles of the present invention apply equally well toalternators and alternator systems having any voltage levels.

Referring now to FIG. 1, an alternator system 10 having output terminals10 a, 10 b includes a three phase alternator 12 having a field currentregulator 14 and a switched-mode rectifier 16 coupled thereto. A fieldcontrol circuit 14 b regulates the output voltage at terminals 10 a, 10b of the alternator system. The field control circuit includes a fieldcurrent regulator 14 and a field controller 14 a. The field currentregulator 14 receives control signals from a field controller 14 a andfunctions to regulate the output voltage at terminals 10 a, 10 b of thealternator system 10. The alternator 12 provides power along threesignal paths 13 a, 13 b, 13 c to the switched-mode rectifier circuit 16.The switched-mode rectifier receives the power from the alternator 12and also receives a duty cycle control signal along path 16 a from aswitched-mode rectifier (SMR) control circuit 18. The SMR controlcircuit 18 receives sensing signals at an input terminal 18 a from aspeed sensor 20 which may be provided as a tachometer for example. Thespeed sensor 20 senses the engine speed or alternator speed and providesa frequency or speed signal to the SMR control circuit 18 along a signalpath 18 a. It should be appreciated that the speed sensor can sense anyparameter or combination of parameters related to ac machine speed (e.g.engine speed, frequency, alternator speed, frequency, alternator backEMF or back EMF frequency, or any quantity from which the appropriateinformation can be observed or estimated) and provide an appropriatesignal to the SMR control circuit. Based upon the frequency or speed ofthe alternator 12 the control circuit 18 provides duty signals alongsignal path 16 a to control the operation (e.g. a duty ratio) of theswitched-mode rectifier 16.

The switched-mode rectifier 16 functions to provide “load matching”between the alternator 12 and a load so that the power level which canbe extracted from the alternator 12 is higher than that which could beachieved with a diode rectifier, for example.

The speed of the alternator 12 provided from speed sensor 20 correspondsto the input signal provided to the SMR control circuit 18 which causesthe switched-mode rectifier 16 to operate at a particular duty ratio.

The output voltage is coupled to a fault protection circuit orcontroller 22 and also to the field controller 14 a. The faultprotection controller 22 operates under fault conditions (e.g. loaddump), and overrides the other controllers 14 a, 18 based on outputvoltage when a load dump occurs. The fault protection controller 22 thusprovides a means for implementing load dump protection. Thus, inclusionof the fault protection controller provides the alternator system 10having a greater degree of circuit protection than can be achieved witha conventional diode rectifier.

In response to detection of a fault condition by the fault protectioncontroller, (e.g. when a significant over-voltage is detected at theoutput terminals of the alternator system) the fault protectioncontroller provides control signals which override control signalsprovided from both the field and switched-mode rectifier controllers.The control signals from the fault protection control circuit cause thefield regulator to drive the field current down and cause theswitched-mode rectifier to limit the load dump transient at the output.That is, in the event of a load dump fault condition the faultprotection controller overrides the field current and duty ratiocommands of the controllers 14 a, 18 such that the field current isdriven down and the switched-mode rectifier limits the load dump voltagetransient at the output terminals 10 a, 10 b. It should be noted thatthe fault protection controller only overrides the other controls for alimited time during a fault.

In operation, the field current regulator 14 act as a primary means forregulating output voltage or current provided from the alternator system10. The field current regulator will also de-energize the field uponcommand from the fault protection control circuit. While the fieldcurrent regulator 14 provides primary control, the switched-moderectifier stage 16 is used as a second control handle to extract levelsof power and performance from the alternator 12 which are relativelyhigh compared with power and performance levels achieved using prior artsystems. The SMR control circuit 18 receives an input signal from thespeed sensor 20 (which may, for example, be provided as a tachometer)and in response to the speed sensor signal, the SMR controller sets theSMR duty ratio so that the maximum power curve is followed as a functionof speed (e.g. see curve 42 in FIG. 3). Additionally, in the specialcase of a load dump (significant over voltage at the alternator systemoutput) being detected, the SMR control circuit will take duty ratiocommands from the fault protection control circuit to limit thetransient seen at the output.

Control of the switched-mode rectifier stage 16 by the fault protectioncontrol circuit provides a greater degree of circuit protection than canbe achieved with a conventional diode rectifier. Thus, the switched-moderectifier 16 provides a valuable degree of additional control over thealternator system output voltage V_(o), while remaining relativelysimple and inexpensive.

As described herein, the approach of using the field current regulator14 as a primary means for regulating alternator system output voltage orcurrent and the control of the switched-mode rectifier stage 16 as asecond control handle to extract levels of power and performance fromthe alternator 12 results in a system which can provide increasedperformance benefits for wound-field alternators having relativelyhigh-reactance values, such as the automotive Lundell-type alternatorsthat are in widespread use today in automobiles.

Thus, unlike prior art systems such as those described in U.S. Pat. No.5,793,625 entitled Boost Converter Regulated Alternator issued August11, 1998 and in an article by G. Venkataramanan, B. Milkovska, V. Gerez,and H. Nehrir, entitled “Variable Speed Operation of Permanent MagnetAlternator Wind Turbines Using a Single Switch Power Converter,” Journalof Solar Energy Engineering—Transactions of the ASME; Vol. 118, No. 4,November 1996, pp. 235-238, the system of the present invention utilizeswound-field alternators and field control as a primary means forregulating alternator system output voltage or current and theswitched-mode rectifier stage 16 as a second control means for achievingload matching to extract higher levels of power from the alternator andfor limiting alternator output voltage during a fault transient.

It should also be appreciated that it has not heretofore been recognizedthat an SMR could be used for the purpose of “load matching” to extracthigher levels of power from an alternator.

It should be appreciated that, the control circuitry associated with thepresent invention can be represented as three sections. One controlsection (the field current regulator controller) receives the outputvoltage as an input and regulates the output voltage by commandingadjustments to the field current. A second control section (theswitched-mode rectifier controller) receives speed as an input andperforms the “load matching” for maximizing power capability byadjusting the duty ratio. The third section (the fault protectioncontrol circuit) receives the output voltage (and possibly speed aswell) as an input. In the event of a load dump fault condition the faultprotection control circuit overrides the field current and duty ratiocommands of the other two controllers (such that the field current isdriven down and the switched-mode rectifier limits the load dump voltagetransient at the output). The fault protection control circuit takeseffect for only a limited time during a fault. As can be seen, in thisimplementation there is only interaction between the field currentregulator and the SMR controller inasmuch as they both take controlinputs from the load dump protection controller during a load dumpfault; the rest of the time the field current regulator and SMRcontroller act independently.

As will be described below in conjunction with FIG. 1B, in someembodiments it may be desirable to use more sophisticatedimplementations in which the field current and the duty ratio arejointly controlled as a function of output voltage and speed. One maythen be able to achieve higher performance (such as higher efficiency)over some parts of the operating range while still achieving both thehigh maximum power transfer capability and output voltage regulation.

As will be described below in conjunction with FIG. 1C, in someembodiments it may be desirable to use a more sophisticatedimplementation in which the switched-mode rectifier duty ratio iscontrolled as a joint function of speed and field current. Using thisapproach one can obtain load matching at all operating points, resultingin higher performance (e.g. high efficiency) at partial load while stillachieving the high maximum power transfer capability.

As will be described below in conjunction with FIG. 1D, in someembodiments it may be desirable to use an implementation in which theswitched-mode rectifier duty ratio is controlled as a function ofmachine back emf (which is related to the product of speed and fieldcurrent). Controlling the switched-mode rectifier as a function ofmachine back emf one can obtain load-matching at all operating points,resulting in higher performance (e.g. high efficiency) at partial loadwhile still achieving the high maximum power transfer capability.

Referring now to FIG. 1A, an alternator system 10′ which may be similarto alternator system 10 described above in conjunction with FIG. 1 isshown having a switched-mode rectifier circuit 16′ comprising a diodebridge 17 and a boost switch set 19 coupled to the diode bridge 17. Thecontrol circuit 18 receives signals from the speed sensor 20 asdescribed above in conjunction with FIG. 1 and provides duty signalsalong path 16 a to control the operation of the boost switch set 19. Theduty cycle of the boost switch set 19 is selected based on the desiredoutput voltage and speed of the alternator 12 such that the alternatorsystem can provide relatively high levels of output power over a rangeof alternator speeds,

Referring now to FIG. 1B, the alternator system 10 described above inconjunction with FIG. 1 is shown in an alternate embodiment whichincludes a state regulator 23 coupled between the field controller 14 a(or alternatively the field current regulator 14) and the SMR controlcircuit 18 (or alternatively the switched-mode rectifier 16). Couplingof the field controller 14A and the SMR control circuit 18 can beadvantageous. For example, by making the duty ratio of the switched-moderectifier 16 a function of both the speed and the field current, one canachieve the same load matching and power improvement for full fieldwhile also achieving higher efficiency over a desired operating range ofspeed and load current or some other design objective. In oneimplementation of such an approach, the field current command could bedetermined to regulate the output voltage, while the SMR duty ratiocould be selected for optimal load matching with respect to theinstantaneous field current command (or field current). In anotherimplementation of such an approach, the field current command and theSMR duty ratio could be selected to jointly control the output voltageand provide load matching while achieving high system efficiency

Referring now to FIG. 2, an alternator system 24 includes a wound-fieldalternator 25 with field control provided by a field current regulator26 coupled to a field controller 26 a. The alternator 25 may be of ahigh-reactance type such as a Lundell alternator. In this particularembodiment, the alternator 25 is modeled as a three-phase y-connectedalternator having a stator modeled as a set of induced voltages vsa,vsb, vsc in series with the alternator leakage inductances Ls. Those ofordinary skill in the art will appreciate of course that for purposes ofthe present invention, it does not matter if the alternator is delta orY connected. The principles of the present invention apply equally wellregardless of the particular manner in which the alternator is connectedand those of ordinary skill in the art would understand how to model thealternator 25 in accordance with the particular manner in which it isconnected. The three phases each have a voltage source, labeled VSA, VSBand VSC that are connected to a common neutral node, N. Each of thethree phases of the alternator 22 has a relatively large associatedinductance designated as Ls. It should be appreciated that while thealternator also includes a resistance, it is not necessary to accountfor the resistance to gain an understanding of the invention.

A switched-mode rectifier 27 coupled to the alternator 25 includes adiode bridge 28 coupled to the alternator 25 as shown. The diode bridgeincludes a plurality of diodes 28 a-28 f which may be provided asconventional diodes typically used in bridge circuits coupled to analternator. Coupled to the diode bridge is a boost switch set or booststage 30 comprising a controlled switch 32 and a diode 34.

It should be noted that the voltage Vx is not only a dc voltage, but italso has a high (switching) frequency ac component. In general, the“local average” of Vx is used to refer to its dc (or low-frequency)component. Because of the large leakage inductances, the alternator 25reacts primarily to the local average of Vx and not the switchingfrequency components. The switch 32 may be provided, for example, as ametal oxide semiconductor field effect transistor (MOSFET). Those ofordinary skill in the art will appreciate how to select the MOSFETswitches having appropriate switch characteristics for each particularapplication. Those of ordinary skill will also appreciate that switchesother than MOSFET switches may be used.

Waveforms of the signals in the switched-mode rectifier 27 and at theoutput of the alternator system are shown in FIGS. 2A and 2B.

A control circuit 36 is coupled to the boost converter 30. The controlcircuit generates a pulse-width modulation (PWM) gate command with dutyratio d. In this case, neither voltage nor current control are used.Instead the duty ratio of the PWM signal is determined as a function ofalternator frequency or speed as provided by the speed sensor 20.

In particular, speed sensor 20 provides a signal S_(w) representative ofthe frequency or speed of the alternator 25 to a conversion circuit 37.It should be appreciated that in alternate embodiments speed sensor 20may derive the signal S_(w) from the speed of the engine (not shown) anengine shaft (not shown) or any other portion of vehicle componentswhich can be related to alternator speed. The conversion circuit 37converts, transforms or otherwise adjusts the signal S_(w) into a signalV_(d) which is subsequently fed through a multiplexer (MUX) 38 to afirst terminal of a comparator 39. The second terminal of the comparator39 is coupled to a reference signal (e.g. a ramp or saw-tooth waveformsignal). The conversion circuit 37 provides the signal V_(d) to thefirst terminal of the comparator 39 through a MUX. The comparator 39compares the signal level of the signal V_(d) to the signal level of thereference signal and provides a signal voltage V_(g) having a high orlow value at the comparator output. Thus, the gate voltage V_(g) isprovided to the switched mode rectifier 30 from the comparator 39 andhas a duty ratio determined by the value of the signal V_(d) and thereference signal.

When the value of the gate voltage Vg is above a first threshold valuethe switch 32 is biased into a first conductive state (e.g. the switch32 is turned on) and when the value of the gate voltage Vg is below thethreshold value the switch 32 is biased into a second conductive state(e.g. the switch 32 is turned off). In this manner, the gate voltageV_(g) thus sets the duty ratio of the switched-mode rectifier 30 at aparticular value.

For example, assume that the reference signal has saw tooth waveform anda voltage level between zero and one volt and also assume that the speedsensor provides the signal S_(w) having value between zero and one(where a value of zero indicates that the alternator has a frequency orrpms of zero and a value of one indicates that the alternator speed isat a maximum value. Thus, if the speed sensor 20 provides a signal Swhaving a signal level corresponding to an alternator speed of about 2400rpms, then from FIG. 2C it can be seen that a duty ratio of about 0.5 isrequired and the circuit 37 would provide V_(d) having a signal level ofabout 0.5 volts.

One exception to duty ratio computation which occurs is during a loaddump fault transient in which case fault protection controller 23determines duty ratio as described above.

It should be noted that the SMR control circuit does not receive adirect feedback signal from the alternator system output. However,during a load dump fault transient, the duty ratio is determined by thefault protection control circuit which does measure the output voltageVo.

In operation of the alternator system 24, field control is used as aprimary means of regulating the output voltage or current of thealternator system 24. At the same time, the switch 32 is turned on andoff at relatively high frequency in a Pulse-Width Modulation (PWM)fashion with duty ratio d. The duty ratio d corresponds to the on timeduty cycle of the FET 32 (defined as the ratio of the on time of the FET32 divided by the sum of the on time and the off time of the FET 32). Inan averaged sense, the boost switch set acts as a dc transformer, with atransformation ratio controlled by the duty cycle.

As with a conventional diode-rectified alternator system, the diodebridge operates in continuous conduction mode (CCM), so that the diode34 is on when the switch 32 is off. The PWM operation of the switch 32and the diode 34 causes the voltage v_(x) to be a pulsing waveform (seeFIG. 2B) with an average value dependent on the output voltage v_(o) andthe duty ratio d. Neglecting device drops, and assuming v_(o), isrelatively constant over a PWM cycle, the local average value of v_(x)denoted as <v_(x)> may be computed as:

 <v _(x)>=(1−d)·v _(o)

Because the PWM frequency is much higher than the ac frequency, andbecause the inductances L_(s) of the alternator 25 are relatively large,the alternator and diode bridge react to the average value of v_(x) in amanner very similar to the manner as they would react to the outputvoltage in a conventional diode-rectified alternator system. As aresult, by controlling the duty ratio d, one has control over theaverage voltage at the output of the bridge, V_(x), to any value belowthe true output voltage of the alternator system, v_(o). Thus, in thesystem of the present invention, the switched-mode rectifier 27 is usedas an additional control handle to extract much higher levels ofperformance from the alternator.

For example, consider that in the system of the present invention, themaximum possible output power of the alternator at a given speed andfield current is determined by the average value of the voltage v_(x),and not by the output voltage v_(o). By adjusting the duty ratio d, thealternator can generate up to its maximum power (across voltage) asalternator speed varies, while supplying a constant output voltage,v_(o). For example, a constant output voltage of 14 V or of 42 V may beprovided.

Referring to FIG. 2C, a curve 41 showing the duty ratio needed toprovide a predetermined output power for a particular commercialalternator with an alternator system output voltage of 42 V is shown.For example, with an alternator system output voltage of 42 V, to obtaina predetermined power output at or close to the maximum power outputwith an alternator speed of 1787 rpm, the switched mode rectifier shouldbe provided having a duty ratio of about 0.6405

The circuit of the present invention can also be operated to provideactive “load-dump” protection.

In a conventional high-reactance alternator system, a seriousover-voltage condition can occur when the output current is suddenlyreduced by a large amount. This important condition, known as “loaddump”, occurs because the voltage drops across the machine reactancesdecrease with the decreased load current, resulting in a much largerfraction of the back emf of the machine being impressed across theoutput. The voltage will be higher than desired until the fieldcontroller can reduce the field current to the new appropriate level (arelatively slow process). In conventional 14-volt automotivealternators, load dump transients as high as 80 V and lasting hundredsof milliseconds can sometimes occur, resulting in serious transientsuppression and circuit protection problems.

In the present invention, however, active load dump protection can beprovided by coordinated control of the field winding and theswitched-mode rectifier via the fault protection controller 23, forexample. Once a load-dump condition is detected, the output voltagetransient can be suppressed by control of the rectifier 27 while fieldcontroller 26 a is used to reduce the back emf of the machine 25 via thefield current regulator 26. In the simplest version of the approach, theboost switch(es) of the rectifier can be turned on continuously and thefield current regulator 26 can be adjusted for deexcitation of the fielduntil the field current and machine currents are at an acceptable level.A more sophisticated version of the approach would adjust thepulse-width modulation of the switched-mode rectifier 27 in concert withthe field current regulator 26 to suppress the transient.

The system of the present invention operates with the diode bridge inCCM, while the boost switch operates under PWM at a relatively highfrequency. The operating characteristics can be derived based on thoseof an uncontrolled bridge as described in V. Caliskan, D. J. Perreault,T. M. Jahns, and J. G. Kassakian, “Analysis of Three-Phase Rectifierswith Constant-Voltage Loads,” 1999 IEEE Power Electronics SpecialistsConference, 1999, pp. 715-720 which is hereby incorporated herein byreference in its entirety. The magnitude of the source (back emf)voltages are controlled by the field current, i_(f), and may beapproximated as:

V _(s) =k·ω·i _(f).

Using well known modeling techniques such as those described in theabove-cited “Analysis of Three-Phase Rectifiers with Constant-VoltageLoads,” and neglecting device drops, circuit resistance terms, and othernonidealities, it can be shown that as long as heavy CCM is maintainedin the line currents, and the condition$V_{s} > {\frac{2}{\pi}\left( {1 - d} \right)v_{o}}$

is met, the average rectified output current will be approximately:

<i _(o)>=3(1−d)(sqrt[V ² _(s)−(4/π²)(1−d)² v ² _(o)])/πωL _(s)

In this equation, one can see that the average output current of therectifier is a function of both the field current i_(f) (whichdetermines V_(s)) and the duty ratio d. In this case, the ac inductorcurrent waveforms i_(a), i_(b), and i_(c) will be continuous andapproximately sinusoidal with fundamental current magnitudes:$I_{s} = \frac{\sqrt{V_{s}^{2} - {\frac{4}{\pi^{2}}\left( {1 - d} \right)^{2}v_{o}^{2}}}}{\omega \quad L_{s}}$

and phases with respect to the ac voltages of:$\varphi = {{atan}{\sqrt{\left\lbrack \frac{\pi \quad V_{s}}{2\left( {1 - d} \right)v_{o}} \right\rbrack^{2} - 1}.}}$

In the alternator system of the present invention, there are multiplehandles on the control of the alternator system. As a result, one canachieve higher levels of output power, efficiency, and transient controlthan can be achieved via conventional means while retaining a relativelysimple circuit structure and control.

While the embodiment of the invention illustrated in FIG. 2 isrelatively simple and effective, those of ordinary skill in the art willnow appreciate that variants, such as that described below inconjunction with FIGS. 4, 6-10, 12, 12A, and 13, based on the sameprinciples exist which have advantages in some applications.

Referring now to FIG. 3 a plot of the calculated output power vs. outputvoltage of a conventional 14 V diode-rectified alternator at constant(full) field current, parameterized by the alternator rpm is shown.Using the switched-mode rectifier approach described above, theeffective bridge voltage seen by the alternator can be varied so thatmore power can be extracted from the alternator across a range ofalternator speeds. The circuit of the present invention allows operationalong the operating locus illustrated as line 42 for extracting apredetermined power over a range of alternator operating speeds as shownin FIG. 3. It should be noted that the actual output voltage may be anyvalue equal to or greater than the effective alternator voltage at thatpoint on the locus. Neglecting device drops, circuit resistance terms,and other nonidealities, it can be shown that the locus of line 42 canbe followed for maximum field current i_(f.max) by selecting a SMR dutyratio equal to:$d = {1 - {\left( \frac{\sqrt{2}\pi \quad k}{4V_{o}} \right) \cdot i_{f,\max} \cdot \omega}}$

where d is constrained in the range 0≦d≦1. Other functional or empiricalrelationships for d may also be utilized to follow the locus of line 42in consideration of circuit nonidealities.

As discussed above, k corresponds to a value which represents arelationship between machine speed and back emf voltage and ωcorresponds to machine speed. The units of k and ω are selected to becompatible (for example if i_(f,max) is in amperes and ω is in rpms thenthe units of k are selected to be V/rpm/amperes.

FIG. 3A shows a plot of the calculated output power vs. output voltageof a conventional 14 V diode-rectified alternator at constant (full)field current, parameterized by the alternator rpm. The operating locifor supplying constant-voltage 14 V and 42 V loads (denoted as lines 48and 50 respectively) via conventional diode rectifier are also shown.

Comparing FIGS. 3 and 3A, it is apparent that the mode of operation ofthe present invention allows much more power to be drawn from themachine at most speeds than is achievable with a diode rectifiersupplying an output voltage of 42 V as shown in FIG. 3A. What makes thispossible is that the switched-mode rectifier provides the necessarycontrolled voltage transformation.

In addition to trends for conventional automotive alternators to providehigher power levels, there will be a need for high-power automotivealternator systems which generate power at higher voltages (e.g. at avoltage of 42 V instead of 14 V). With the present invention, evenpresent 14 V machine designs are suitable for high-power operation at 42V output since only the rectifier stage and controls need to be changed,as can be inferred from FIG. 3. Manufacturing both 14 V and 42 Vversions of an alternator on the same manufacturing line would bepossible and would result in savings of time and expense. Thus, thepresent invention is timely for meeting the demands of higher power andhigher voltage alternators in the automotive industry while remainingwithin the existing manufacturing framework and overcoming the presentday load dump transient problem.

Because of its voltage transforming effect, the switched-mode rectifierallows much higher levels of current and power to be extracted from thealternator than would be possible with a conventional diode rectifier.At the same time, the output current or voltage can be efficientlyregulated to any value below the maximum via field control.

This contrasts with the approach taken in prior art systems such asthose described in U.S. Pat. No. 5,793,625 and the article entitled“Variable Speed Operation of Permanent Magnet Alternator Wind TurbinesUsing a Single Switch Power Converter,” in which control of theswitching rectifier alone is used to regulate output current or voltage.Unlike the low-reactance case considered in such prior art systems,regulating the output using the switching rectifier in high-reactancemachines such as the Lundell alternator can have serious disadvantages.For example, considering the alternator operating characteristics ofFIG. 3A for an alternator supplying an output voltage of 42 V at a speedof 6000 rpm, maximum output power at 42 V output would be achieved byusing a duty ratio d=0. Reducing the output power by control of theswitched-mode rectifier can only be achieved by increasing the dutyratio. In addition to reducing the output current, this would have theeffect of reducing the effective alternator voltage v_(x), and in turnincreasing the machine and semiconductor device currents. (In thelimiting case, zero output current and power is achieved at a duty ratiod=1, with the machine operating at high current into a short circuitprovided by the switching rectifier.) As a result, for the machinecharacteristics of FIG. 3A, regulating the output using only a switchingrectifier can result in very poor part-load efficiency. In the presentinvention, however, the output power can be efficiently regulated belowthe maximum using field control. The problem presented by prior artsystems does not exist.

Referring now to FIG. 4, an alternator system 52 includes the alternator25 and a field current regulator and field controller 26, 26 a whichoperate in the same manner as the alternator, field current regulatorand field controller described above in conjunction with FIG. 2. Thealternator 25 is coupled to a switched-mode rectifier circuit 54.Although not here shown, the alternator system 52 can optionally includea fault protection circuit and a state regulator as described above inconjunction with FIGS. 1, 1B, 1C, and 1D, respectively.

In this particular embodiment, rectifier circuit 54 includes a bridgecircuit comprising a plurality of diodes 56 a-56 f and having a PWMstage 58 integrated therein. The PWM stage 58 includes a plurality ofswitches 58 a, 58 b, 58 c which are gated on and off together in a PWMfashion with duty ratio d via control signals provided by controlcircuit 36. Operation in this mode yields waveforms i_(a), i_(b), i_(c),and i_(o) that are essentially the same as those of the circuit of FIG.2, along with the same duty-ratio control characteristics.

One advantage of this embodiment is that there are fewer series devicedrops than in the circuit of FIG. 2, thus resulting in lower losses.Furthermore, if MOSFETs are used for the active switches 58 a-58 c,external antiparallel diodes 56 d-56 f can be eliminated, andsynchronous rectification can be implemented for those devices as well(using a additional conventional control circuitry well known to thoseof ordinary skill in the art), thereby further reducing losses.

It should be noted that the manner in which the PWM stage 58 is operatedis different than in prior art systems. In prior art systems, the dutyratio of the PWM stage is determined as a function of output voltage inorder to regulate the output voltage. In the present invention, however,the duty ratio of the PWM stage is determined as a function ofalternator speed, in order to allow “load matching” (e.g. following thelocus 42 in FIG. 3 so that maximum power can be extracted from thealternator 25).

Referring now to FIG. 1C, an alternator system 10 having outputterminals 10 a, 10 b includes a three phase alternator 12 having a fieldcurrent regulator 14 and a switched-mode rectifier 16 coupled thereto.The field current regulator 14 receives control signals from a fieldcontroller 14 a and functions to regulate the output voltage atterminals 10 a, 10 b of the alternator system 10. The alternator 12provides power along three signal paths 13 a, 13 b, 13 c to theswitched-mode rectifier circuit 16. The switched-mode rectifier receivesthe power from the alternator 12 and also receives a duty cycle controlsignal along path 16 a from a switched-mode rectifier (SMR) controlcircuit 18. The SMR control circuit 18 receives sensing signals at aninput terminal 18 a from a speed sensor 20 which may be provided as atachometer for example. The speed sensor 20 senses the engine speed oralternator speed and provides a frequency or speed signal to the SMRcontrol circuit 18 along a signal path 18 a. It should be appreciatedthat the speed sensor can sense any parameter or combination ofparameters related to ac machine speed (e.g. engine speed, frequency,alternator speed, frequency, alternator back EMF or back EMF frequency,or any quantity from which the appropriate information can be observedor estimated) and provide an appropriate signal to the SMR controlcircuit. The SMR control circuit 18 receives sensing signals at an inputterminal 18 b from the field current regulator 14 (or alternatively fromthe field controller 14 a). The sensing signal at input terminal 18 bcontains information about the field current of the machine, and may beprovided as a voltage proportional to actual field current. It should beappreciated that the sensing signals provided by the field regulator 14to the SMR control circuit 18 at input terminal 18 b may be any signalrelated to field current (e.g. field current, average field voltage,field regulator duty ratio, commanded field current, commanded fieldvoltage, magnetic field strength due to the field winding, etc.) or anyquantity from which the appropriate information about the field currentcan be observed or estimated.

Based upon the frequency or speed of the alternator 12 and the fieldcurrent of the alternator 12, the control circuit 18 provides dutysignals along signal path 16 a to control the operation (e.g. a dutyratio) of the switched-mode rectifier 16. The switched-mode rectifier 16functions to provide “load matching” between the alternator 12 and aload so that the power level which can be extracted from the alternator12 is higher than that which could be achieved with a diode rectifier,for example. The speed of the alternator 12 provided from speed sensor20 and the field current of the alternator 12 provided from the fieldcurrent regulator 14 correspond to the input signals provided to the SMRcontrol circuit 18 which causes the switched-mode rectifier 16 tooperate at a particular duty ratio.

Referring now to FIG. 2D, an alternator system 24 which may be similarto alternator system 10 described above in conjunction with FIG. 1Cincludes a wound-field alternator 25 with field control provided by afield current regulator 26 coupled to a field controller 26 a. Aswitched-mode rectifier 27 coupled to the alternator 25 includes a diodebridge coupled to the alternator 25 as shown. Coupled to the diodebridge is a boost switch set or boost stage 30 comprising a controlledswitch 32 and a diode 34. A control circuit 36 is coupled to the booststage 30. The control circuit generates a pulse-width modulation (PWM)gate command with duty ratio d. In this case, the duty ratio of the PWMsignal is determined as a joint function alternator frequency or speed(as provided by speed sensor 20) and alternator field current (asprovided by field current regulator 26).

In particular, speed sensor 20 provides a signal S_(w) proportional tothe speed or frequency of the alternator to a conversion circuit 37.Field current regulator 26 provides a signal S_(if) proportional to thefield current to the conversion circuit 37. The conversion circuit 37combines and transforms these signals into a signal V_(d) which issubsequently fed through a multiplexer (MUX) 38 to a first terminal of acomparator 39. The second terminal of the comparator 39 is coupled to areference signal (e.g. a ramp or saw-tooth waveform signal). Theconversion circuit 37 provides the signal V_(d) such that the gatevoltage V_(g) is provided to the switched-mode rectifier 30 from thecomparator 39. The gate voltage V_(g) sets the duty ratio of theswitched-mode rectifier at a particular value. Essentially thealternator speed is fed to the circuit 37 and the field current isprovided from field current regulator 26. Thus, in accordance with therelation between the field control and the speed (as shown in FIG. 2Efor example, circuit 37 provides a signal V_(d) having a voltage levelwhich results in the switched mode rectifier having the desired dutyratio.

In this embodiment of the invention, the duty ratio d is selected toprovide “load matching” at every level of alternator speed and fieldcurrent. This allows the alternator to achieve much higher levels ofoutput power than is conventionally achieved, and also provides for veryefficient operation over a wide range of partial load conditions.Neglecting device drops, circuit resistance terms, and othernonidealities, it can be shown that the load matching locus can befollowed for any field current i_(f) by selecting a SMR duty ratio equalto:$d = {1 - {\left( \frac{\sqrt{2}\pi \quad k}{4V_{o}} \right) \cdot i_{f} \cdot \omega}}$

where d is constrained in the range 0≦d≦1. The value of V_(d) can thusoptionally be provided by circuit 37 by the circuit implementing afunction equal or equivalent to one minus a scaled version of theproduct of the field current and speed signals. (It should be recognizedthat similar approaches such as controlling the complement of the dutyratio d′=1−d also result in very simple control circuitimplementations.) Other functional or empirical relationships for d ord′ based on alternator speed and field current can also be utilized tofollow the load matching locus in consideration of circuitnonidealities, or to achieve other design objectives. It should berecognized that the boost rectifier structure of FIG. 4 can be equallywell employed in this embodiment.

In comparison to the embodiment of FIG. 2, for example, this embodimentallows the same high maximum output power to be achieved. Thisembodiment also allows load matching to be achieved at part-loadoperation, when less than maximum field current is required. In thispart of the operating range, lower levels of field current and higherlevels of efficiency can be achieved with this embodiment at the expenseof a slight increase in controller complexity (e.g. a feedback signalpath from rectifier 26 to circuit 37).

Referring now to FIG. 2E, a series of curves 37 a-37 d showing the dutyratio needed to provide load match conditions at a variety of alternatorspeeds are shown. By selecting an alternator speed-duty ratio pair whichfall on one of the curves 37 a-37 d, the selected duty ratio d provides“load matching” at the selected level of alternator speed and fieldcurrent. An alternator operating along these curves achieves levels ofoutput power which are relatively high compared with thoseconventionally achieved and also provides for very efficient operationover a wide range of partial load conditions.

For example, assuming an alternator operates at fifty percent of fullfield current and at an alternator speed of about four thousand rpm, byselecting the duty ratio to be output 0.64, the alternator operates at a“load match” condition at which the alternator provides power output ator close to the maximum possible power output for the given conditions.

Referring now to FIG. 1D, an alternator system 10 having outputterminals 10 a, 10 b includes a three phase alternator 12 having a fieldcurrent regulator 14 and a switched-mode rectifier 16 coupled thereto.The field current regulator 14 receives control signals from a fieldcontroller 14 a and functions to regulate the output voltage atterminals 10 a, 10 b of the alternator system 10. The alternator 12provides power along three signal paths 13 a, 13 b, 13 c to theswitched-mode rectifier circuit 16. The switched-mode rectifier receivesthe power from the alternator 12 and also receives a duty cycle controlsignal along path 16 a from a switched-mode rectifier (SMR) controlcircuit 18. The SMR control circuit 18 receives sensing signals at aninput terminal 18 a from a back emf sensor 20.

The back emf sensor 20 senses the back emf of the machine, and providesa signal representative of a signal characteristic of the back emf (e.g.the magnitude or frequency of the back emf) to control circuit 18 alonga signal path 18 a. It should be appreciated that the back emf sensorcan sense any parameter or combination of parameters related to acmachine back emf (e.g. back emf waveform, back emf waveform magnitude,field winding magnetic field strength, or any quantity from which theappropriate information can be observed or estimated) and provide anappropriate signal to the SMR control circuit. The back emf sensor maybe implemented as an additional armature winding or set of windings anda peak detector circuit, for example.

Based upon the back emf or back emf magnitude of the alternator 12, thecontrol circuit 21 provides duty signals along signal path 16 a tocontrol the operation (e.g. a duty ratio) of the switched-mode rectifier16. The switched-mode rectifier 16 functions to provide “load matching”between the alternator 12 and a load so that the power level which canbe extracted from the alternator 12 is higher than that which could beachieved with a diode rectifier, for example. The back emf or back emfmagnitude of the alternator 12 provided from back emf sensor 20corresponds to the input signal provided to the SMR control circuit 18which causes the switched-mode rectifier 16 to operate at a particularduty ratio.

Referring now to FIG. 2F, an alternator system 24 which may be similarto alternator system 10 described above in conjunction with FIG. 1Dincludes a wound-field alternator 25 with field control provided by afield current regulator 26 coupled to a field controller 26 a. Aswitched-mode rectifier 27 coupled to the alternator 25 includes a diodebridge coupled to the alternator 25 as shown. Coupled to the diodebridge is a boost switch set or boost stage 30 comprising a controlledswitch 32 and a diode 34. A control circuit 36 is coupled to the booststage 30. The control circuit generates a pulse-width modulation (PWM)gate command with duty ratio d. In this case, the duty ratio of the PWMsignal is determined as a function of alternator back emf or back emfmagnitude (as provided by back emf sensor 20).

In particular, back emf sensor 20 provides a signal S_(EMF) proportionalto the back emf or back emf magnitude of the alternator to a conversioncircuit 37. The conversion circuit 37 converts or transforms this signalinto a signal V_(d) which is subsequently fed through a multiplexer(MUX) 38 to a first terminal of a comparator 39. The second terminal ofthe comparator 39 is coupled to a reference signal (e.g. a ramp orsaw-tooth waveform signal). The conversion circuit 37 provides thesignal V_(d) such that the gate voltage V_(g) is provided to theswitched-mode rectifier 30 from the comparator 39. The gate voltageV_(g) sets the duty ratio of the switched-mode rectifier at a particularvalue.

In this embodiment of the invention, the duty ratio d is selected toprovide “load matching” at every level of back emf. This allows thealternator to achieve much higher levels of output power than isconventionally achieved, and also provides for very efficient operationover a wide range of partial load conditions. Neglecting device drops,circuit resistance terms, and other nonidealities, it can be shown thatthe load matching locus can be followed for any machine back emf(source) voltage magnitude V_(s), (or equivalently any speed and fieldcurrent) by selecting a SMR duty ratio equal to:$d = {1 - {\left( \frac{\sqrt{2}\pi}{4V_{o}} \right) \cdot V_{s}}}$

where d is constrained in the range 0≦d≦1. The value of the signal V_(d)can thus optionally be implemented in the block 37 as one minus a scaledversion of the back emf magnitude. (It should be recognized that similarapproaches such as controlling the complement of the duty ratio d′=1−dalso result in very simple control circuit implementations.) Otherfunctional or empirical relationships for d or d′ based on alternatorback emf or back emf magnitude can also be utilized to follow the loadmatching locus in consideration of circuit nonidealities, or to achieveother design objectives. It should also be noted that the boostrectifier structure of FIG. 4 can be equally well employed in thisembodiment.

In comparison to the embodiment of FIG. 2, for example, this embodimentallows the same high maximum output power to be achieved. Thisembodiment also allows load matching to be achieved at part-loadoperation, when less than maximum field current is required. In thispart of the operating range, lower levels of field current and higherlevels of efficiency can be achieved with this embodiment. In comparisonto the embodiment of FIG. 2d, this embodiment allows similar levels ofoutput power and part-load load matching to be achieved but requiresdifferent sensing and control circuitry.

Referring now to FIG. 2G, an alternator system 24 which may be similarto alternator system 10 described in conjunction with FIG. 1B includes awound-field alternator 25 with field control provided by a field currentregulator 26 coupled to a field controller 26 a. A switched-moderectifier 27 coupled to the alternator 25 includes a diode bridgecoupled to the alternator 25 as shown. Coupled to the diode bridge is aboost switch set or boost stage 30 comprising a controlled switch 32 anda diode 34.

A control circuit 36 coupled to the boost stage 30 and to the fieldcontroller 26 a generates a pulse width modulation (PWM) gate commandwith duty ratio d. The control circuit also generates a command, S_(f),to the field controller. The signal S_(f) provides the control commandfor the field winding, and may represent the fraction of full field toapply or alternatively the duty cycle of the field current regulator touse, or an equivalent control signal. The signals S_(f) and d aredetermined based on the alternator system output voltage and thealternator speed.

In particular, compensator 50 generates a control signal X based on thedesired output voltage and the actual output voltage. Compensator 50 mayrepresent a gain or transfer function working on the difference betweendesired and actual output voltage or be some other function of actualand desired output voltage. This signal X is fed firstly to a limiter 51that generates a signal S_(f) that is a replica of X except that itsvalue is limited between 0 and 1. The signal S_(f) is provided as acontrol command to the field current controller, and may represent thefraction of full field to use or field regulator duty cycle to use orsome equivalent signal. Signal X is secondly used to generate a signalX′ which is X minus 1. X′ is fed to controlled limiter 52.

The controlled limiter 52 generates a signal V_(d) that is a replica ofX′ except that it is limited between zero and a value V_(dm). The valueV_(dm) is generated in the manner described below: Speed sensor 20generates a signal, S_(T), that is proportional to alternator speed, andprovides this signal to conversion circuit 37. The conversion circuit 37transforms this signal into a signal V_(dm) which is provided tocontrolled limiter 52. The conversion circuit 37 calculates the dutycycle for matched operation d_(m) at full current based on thealternator speed. This may be an empirically-derived function oralternatively may be calculated as:$d_{m} = {1 - {\left( \frac{\sqrt{2}\pi \quad {ki}_{f,\max}}{4V_{o,{ref}}} \right) \cdot \omega}}$

Alternatively still, the relationship may be determined through otherfunctional or empirical relationships.

In this embodiment of the invention, the SMR duty ratio d and the fieldcontroller command S_(f) are jointly determined as a function of outputvoltage in a manner that allows the high-power operation of the machineto be achieved while reaching a very high part-load efficiency. In manyautomotive alternators, over much of the speed range the conductionlosses in the stator windings and the semiconductor devices represent adominant portion of the alternator losses. The control scheme of thisembodiment reduces the losses incurred by those elements by ensuringthat the alternator system generates the needed output power utilizingthe lowest stator and device currents possible. This is done bycontrolling the SMR so that the alternator always sees the largesteffective voltage V_(x) that can be used for that level of output power.For a boost-type SMR, this is done by ensuring that the lowest dutyratio possible (for the required level of output power) is always used.The system of FIG. 2G does this by ensuring that the duty ratio willonly be raised above zero when full field current at zero duty ratio isnot sufficient to provide the needed output power. In this case the dutyratio will only be raised by an amount necessary to provide the neededoutput power or to reach the load-matched condition, whichever is lower.The control method thus allows very high efficiency control of thesystem while providing a simple, inexpensive control structure. Those ofordinary skill in the art will recognize that similar embodimentsoptionally using other rectifier types may also be used to achieve thiscontrol mode (that is, ensuring that the minimum stator and devicecurrents are used for generating the necessary output power).

Referring now to FIG. 5, a flow diagram illustrating the steps to designan alternator in accordance with the present invention are described.First, as shown in step 60 a suitable cruising speed to design for isselected.

Second, as shown in step 62 the number of alternator stator windingturns is selected such that the peak of the output power versus outputvoltage curve (for diode rectification at full field) at the designspeed reaches its maximum at the desired output voltage.

Thus, in accordance with the present invention conventional alternatordesigns can be adjusted by rewinding the alternator with a number ofturns so that at or near maximum cruising speed, maximum power isachieved at the desired output voltage. This is in contrast toconventional designs in which the maximum power is achieved at thedesired output voltage at idle speed.

It should be noted that there are other more sophisticated modificationsthat can be made to an alternator for good operation in accordance withthe present invention, such as reoptimizing the magnetic and thermaldesign of the alternator. Fully reoptimizing the alternator for this newapproach (e.g. redesigning the magnetics, thermal design, etc.) isexpected to yield good results for alternators providing output at anyoutput voltage.

While FIGS. 2, 2D, 2F, 2G, and 4 show versions of the invention withsimple “boost-type” switched-mode rectifiers, other simple rectifierstructures can also be used to implement the new invention. Thus,several alternative rectifier structures are described below inconjunction with FIGS. 6-10.

For example, FIGS. 6, 7 and 8 illustrate alternate embodiments of theinvention with SEPIC, Cuk, and current-fed push-pull versions of theswitching rectifier. Isolated versions of the rectifiers, such asillustrated in FIGS. 9 and 10, can also be used. In these embodiments,the duty ratio control laws will be different than those of the systemsusing a boost rectifier. However, appropriate duty ratio control lawsfor achieving load matching with these rectifier circuits are easilyderived by determining the duty ratio necessary to properly match theaverage alternator voltage to the output voltage Vo as a function ofspeed and field current or equivalently as a function of back voltage.Several examples have been provided above for particular embodiments. Itshould be appreciated, however, that particular relationships betweenthe average alternator voltage and output voltage Vo, speed, fieldcurrent, and back voltage will depend upon particular embodiments andthat the examples provided herein are exemplary only and should not beconstrued as limiting the scope of the general concept of the inventionin any way.

The ability to achieve load dump suppression is also retained in thesealternative rectifier structures. The SEPIC-based rectifier used in FIG.6 can provide an effective transformation ratio greater than or lessthan one by varying duty ratio, as can the rectifiers of 7, 9, and 10.The rectifier of FIG. 8 can also do this, but only through properselection of the transformer turns ratio. The Cuk-based rectifier usedin FIG. 7 provides smoothed output current and natural output voltageinversion, in cases where that is desirable. The push-pull basedrectifier used in FIG. 8 provides isolation and additional degrees ofdesign freedom through incorporation of a high-frequency transformer, asdo the rectifiers used in FIGS. 9 and 10.

It should be noted that while all of these rectifier topologies havebeen proposed previously (such as in an article entitled “Single-Switch3φ PWN Low Harmonic Rectifiers,” by E. H. Ismail and R. Erickson in theIEEE Transactions on Power Electronics Vol. 11, No. 2, March 1996, pp.338-346), they have not been applied in the manner of the presentinvention.

In the present invention these switching rectifiers are used to providean additional control handle to extract much higher levels ofperformance and power from the alternator, as described previously forthe boost-derived topologies of FIGS. 2 and 4. By adjusting theswitching duty cycle d, the alternator can generate up to its maximumpower (across voltage) as speed varies while supplying a constant outputvoltage; this is possible because the switched-mode rectifier makes thenecessary controlled voltage transformation. It should be noted that theduty ratio control laws used to achieve the desired effect are differentfor these topologies than for the boost-derived rectifiers, but areeasily determined. Use of these alternative rectifier structures alsopreserves other advantages of the invention such as the ability toimplement load dump suppression.

FIGS. 11-13 illustrate Dual-Output Alternator Configurations. Adual-output alternator system has two outputs which must be regulated. Anumber of methods for achieving this are possible.

One prior art approach often-considered for a dual-output rectifiertopology is illustrated in FIG. 11. In this system field control is usedto regulate the high-voltage output (fed via the high-side diodes),while firing angle control (within the machine electrical cycle) is usedto regulate the low-voltage output (fed via the thyristors).

Referring now to FIG. 12, a dual output alternator system 70 includes analternator 25 having a field current regulator and field controller 26,26 a coupled thereto. Alternator output terminals 25 a, 25 b, 25 c havea first set of switching elements 72 a, 72 b, 72 c generally denoted 72coupled thereto. In this particular embodiment, the switching elements72 are provided as MOSFETs having a first terminal coupled to arespective one of the alternator output terminals 25 a, 25 b, 25 c,second terminals coupled to a reference potential (here shown as ground)and a control terminal which receives control signals from a controlsystem 74.

Also coupled to the alternator output terminals are a series of diodes76 a, 76 b, 76 c with each of the diodes having a first terminal coupledto the alternator output terminals 25 a, 25 b, 25 c and the firstterminals of the MOSFETs 72 a, 72 b, 72 c and second terminals coupledto an output terminal of the alternator system.

The dual output alternator system 70 also includes a plurality ofthyristors 78 a, 78 b, 78 c. It will be recognized by those of ordinaryskill in the art that the thyristor devices may be replaced by differentswitching elements that can achieve the desired effect. Such alternativeimplementations include the use of diodes in series with MOSFET switches(or a set of three diodes with one MOSFET connected to their cathodes)or the use of MOS-controlled thyristors (MCTs). The thyristors 76 a, 76b, 76 c receive control signals from the control system 74 which biasesthe thyristors between their conduction and non-conduction states. Thus,in this particular embodiment, the alternator 25 provides power to afirst high voltage bus through switched-mode rectifier 71 and alsoprovides power to a second low voltage bus through thyristors 78 a-78 c.

In this particular embodiment, the control system 74 includes a controlprocessor 80 which receives input signals corresponding to speed, andthe dual output voltages V₀₁, V₀₂ and provides signals to a firstduty/firing controller 82, a second duty/firing controller 84 and afield controller 86. It should be appreciated that although the controlsystem 74 is here shown provided from a plurality of differentcontrollers and processors, it should be appreciated that the controlsystem could also be implemented as a single controller or processor 74which provides all the functions performed by processor 80 andcontrollers 82, 84, 86.

The processor 80 receives the output voltage signals V_(o1), V_(o2) andas well as a speed signal and determines a duty ratio and pre-processesthe voltages V_(o1), V_(o2) and provides the pre-processed signals tothe field controller. The first duty/firing controller 82 providescontrol signals to the gate terminal of the thyristors 78 a, 78 b, 78 cto bias the thyristors into their conduction states. The thyristors turnoff when a reverse voltage is applied across the anode and cathode ofthe thyristor.

The control system 74 controls the dual output alternator system 70 asfollows: within each switching cycle MOSFETs Q_(x), Q_(y), Q_(z) (orother switching devices having appropriate characteristics) are gated ontogether for a duty ratio d. After they are turned off, the machinecurrent will flow through one or more diodes D_(x), D_(y), D_(z),feeding current to the high-voltage output. After a controlled delay d₂,thyristors 78 a-78 c are fired, which redirects the machine current tofeed the low-voltage bus. (Alternative control methods are possible inwhich the thyristors are only fired on some switching cycles.) Threecontrol handles are available: the duty cycle of the MOSFETs Q_(x),Q_(y), Q_(z) (gated together) provides one control handle, the seconddelay of the thyristors d₂ provides a second, and field control providesa third control handle. In alternative control methods where thethyristors are only fired on some switching cycles, the fraction ofcycles on which the thyristors are fired can replace the controlleddelay d₂ as the last control handle. Together, these control handles aresufficient to allow regulation of the two outputs while simultaneouslycontrolling the local average voltage the machine sees so that maximumpower can be extracted.

For example, one could control field current so that the total currentneeded at both outputs V₀₁, V₀₂ is supplied by the ac machine 25,control the delay d2 so that the total current is split between the twobusses as desired to satisfy the two outputs and control the SMR dutyratio based on the speed, output voltages (or their desired values),commanded field current, and d2 so that the load matching requirementfor high power is met. By analogy to the discussion presented above forthe single output case, and described above in conjunction with FIGS.1-4, the SMR duty ratio is selected so that the time average voltagepresented at the alternator machine terminals results in the peak ofoutput power vs. alternator voltage being selected.

Another important advantage of this dual-output alternator system 70 isthat the PWM operation of the MOSFETs allows switching of current backand forth between the diode and thyristor outputs to occur at theswitching frequency. In contrast, in the prior art system of FIG. 11,switching of current back and forth between the diode and thyristoroutputs occurs at a low multiple of the machine frequency. Thisdramatically reduces the size of filters used on the outputs toattenuate ripple. Although not explicitly shown in FIG. 12, it should beappreciated that the dual output alternator system 70 could also includea fault protection control circuit of the type described above inconjunction with FIGS. 1, 1A, 1B and 2. Thus, the circuit architectureof the system 70 also preserves the ability to control load dumptransients.

Also described are other dual-output extensions to the invention. If aswitched-mode rectifier stage with a transformer is used (such as theisolated Cuk, isolated SEPIC, or push-pull based rectifier as shown inFIGS. 8-10 above), a second output can be supplied through the use of anadditional transformer winding. Because the transformers in theseversions of the present invention are designed for switching-frequencyoperation, they can be relatively small and inexpensive. Rectificationon the second winding may be provided with thyristors (maintaining thesame number of control handles) or with diodes if the transformer turnsratios are selected properly and the resulting cross-regulation betweenthe two output voltages is acceptable.

Referring now to FIG. 12A, a dual output alternator system 70′ includesan alternator 25 having a field current regulator 26 coupled thereto.Alternator output terminals 25 a, 25 b, 25 c have a first set ofswitching elements generally denoted 73 coupled thereto. In thisparticular embodiment, the switching elements 73 are provided as aplurality of diodes coupled in a full bridge configuration. The bridgeis coupled to a switched mode converter 119.

In particular, terminal e of the diode bridge 73 is coupled to firstterminals of switching elements 58 a′, 58 b′ and terminal d of the diodebridge is coupled to a transformer 120. In this particular embodiment,the switching elements 58 a′, 58 b′, are provided as MOSFETs each havingfirst terminals (here corresponding to source terminals) coupled toterminal e of the diode bridge 73.

The transformer 120 includes a primary transformer winding 122 having N1turns. Terminal e of the diode bridge is coupled to the center tap ofthe primary transformer winding 122 while second terminals (here drainterminals) of the switches 58 a′ and 58 b′ are coupled to opposite endsof the primary transformer winding 122. The transformer 120 alsoincludes secondary windings 124, 126 having turns N2, N3 respectively.The secondary winding 124 is connected with diodes 128 and 129 to outputvoltage V₀₁ and secondary winding 126 is connected with diodes 130 and131 to the output voltage V₀₂. The turns ratios N1/N2 and N1/N3 arechosen to have the desired voltage levels V₀₁ and V₀₂.

The switches 58 a′, 58 b′ also have third or control terminals (heregate terminals) which receive control signals from a control system 74′.

In this particular embodiment, the control system 74′ includes a controlprocessor 80 which receives input signals corresponding to speed and theoutput voltage V₀₁. It should be appreciated that although the voltageV₀₁ is used in this particular example, the voltage V₀₂ couldalternatively be used. Thus, the control processor 80 can receive inputsignals corresponding to speed and one of the output voltages V₀₁ orV₀₂.

The control processor 80′ provides control signals to duty controller84′ and field controller 86′. It should be appreciated that although thecontrol system 74′ is here shown provided from a plurality of differentcontrollers and processors, it should be appreciated that the controlsystem 74′ could also be implemented as a single controller or processor74′ which provides all the functions performed by processor 80 andcontrollers 84′ and 86′.

In operation, the control system 74′ receives the speed signal and theoutput voltage signal V_(o1). The control system 74′ determines a dutyratio for the switches 58 a′, 58 b′ from the speed signal and providesswitch control signals to the switches 58 a, 58 b through the dutycontroller 84′. The control system 74′ also pre-processes the voltageV_(o1), and provides the pre-processed signal to the field controller86′ which in turn provides field control signals to the field currentregulator 26 which controls the output voltage level provided by thealternator 25.

The control system 74′ controls the dual output alternator system 70′ asdescribed below. Each of the MOSFETs 58 a′, 58 b′ (or other switchingdevices having appropriate characteristics) receive control signals fromthe duty controller 84′. The control signals cause the MOSFETs 58 a′, 58b′ to be either on or off.

During at least a portion of the switching cycle, the FET 58 a′, isturned on, and FET 58 b′ is turned off. In this case, current flowsthrough section A of the primary winding 122 and through MOSFET 58 a.Current flows in the secondary windings 124, 126 through diodes 128 and130 respectively to deliver power to the output voltages V₀₁, V₀₂.

During at least another portion of the switching cycle, the FET 58 a′,is turned off, and FET 58 b′ is turned on. In this case, current flowsthrough section B of the primary winding 122 and through MOSFET 58 b′.Currents flow in the secondary windings 124, 126 through diodes 129 and131 respectively to deliver power to the output voltages V₀₁, V₀₂.

It should be noted that switching profile of MOSFETS 58 a′ and 58 b′must always provide a flow path for current directed into the center tapof the primary winding 122 of transformer 120. As a result, it must beensured that switches 58 a′ and 58 b′ are never off simultaneously. Toguarantee that MOSFETs 58 a′ and 58 b′ are never off together, there areintervals during the switching interval where both switches are on.These intervals are utilized for the switches to transition from thereon states to their off states, or vice versa. For example, consider thecase where MOSFET 58 a′ is on and MOSFET 58 b′ is off Before turningMOSFET 58 a′ off and MOSFET 58 b′ on, MOSFET 58 b′ is turned on. Duringthe interval both MOSFETs are on, the current flowing into the centertap of primary winding 122 of transformer 120 divides equally betweensection A and section B of primary winding 122. Furthermore, during thisinterval no current flows in the secondary windings 124 and 126 oftransformer 120. The overlap on-time of the MOSFETs is chosen to allowsmooth switch state transitions (on to off or off to on) for theswitches while accounting for the finite state transition times forpractical devices chosen for the particular application. The selectedoverlap time also provides the mechanism for controlling the averagevoltage across terminals d and e.

Together, these control handles are sufficient to allow regulation ofthe two outputs V₀₁, V₀₂ while simultaneously controlling the localaverage voltage the machine sees so that maximum power can be extracted.

The output voltage V₀₁ is proportional to the turns ratio N2/N1 andoutput voltage V₀₂ is proportional to N3/N1. It can be shown that theconstant of proportionality for V₀₁ and V₀₂ is identical and is afunction of the duty ratio of the switches and the local average of thevoltage across terminals d and e. As a result, it follows that voltagesV₀₁ and V₀₂ are related by the turns ratio N2/N3 and are not bothindependent variables. Therefore, it suffices to use either V₀₁ or V₀₂as the variable to be controlled in our control system 74′. With one ofthe two voltages chosen to be controlled, choosing the correct turnsratio N2/N3 indirectly controls the other variable.

Referring now to FIG. 13, a dual-output alternator system 90 implementedusing dual-wound alternator 92 is shown. In such alternator systems,separate machine windings 93 a, 93 b are used with separate rectifiers94 a, 94 b to supply two outputs 96 a, 96 b to which are coupled loads98 a, 98 b. The present invention can be extended to this case by usinga simple switched-mode rectifier with the appropriate control for one orboth of the rectifiers 94 a, 94 b in the dual-wound alternator system.In addition to allowing increased power output and load dump control,this variant of the new invention allows more freedom in the design ofthe alternator machine 92 because of the degrees of freedom provided byduty-cycle control of the rectifier(s) 94 a, 94 b.

Referring now to FIG. 14, an alternator system 100 which includescircuitry to enable a method for charging a battery 102 at the output ofa switched-mode rectifier 54 includes a connecting system 104 forselectively connecting the positive terminal of a charging source (notshown) to an ac machine.

In this particular example, connecting system 104 is coupled to themachine neutral point. It should, however, be appreciated that theconnecting system could also be coupled to portions other than themachine neutral. For example, it could be connected to one of the phaseoutputs. In this manner all or a portion of the machine inductances canbe utilized with all or a portion of the SMR as a switching powerconverter to charge the battery from the charging source. Thus, in thecase where the connecting means is coupled to one of the phase outputs(rather than the machine neutral) the corresponding portion of the SMRcoupled to that particular output would not be used (e.g. turned off).

It should be appreciated that in those embodiments described hereinwhich include a jump charging circuit a switching converter or aninverter could be substituted for the switched-mode rectifier 54. Thus,in some embodiments it may be desirable or even necessary to replace theswitched-mode rectifier 54 with a switching converter or an inverter. Inthose applications in which a switching power converter or inverter isused, the SMR control circuit is replaced by appropriate controlcircuitry.

Connecting system 104 can be provided, for example, as a connector or aswitch (such as a mechanical switch, relay, or semiconductor switch), orby another connecting means. In one embodiment, the battery 102 isprovided having a first voltage level and the voltage source coupled toconnecting system is provided having a second voltage level which islower than the voltage level of the battery 102. Thus, in this case, thebattery 102 corresponds to a high voltage source and the charging sourcecoupled to connecting system 104 corresponds to a low voltage source.

The negative terminal of the low-voltage source is connected to systemground as is the negative terminal of the high-voltage battery 102. Inthis configuration the alternator machine inductances in conjunctionwith the switched-mode rectifier can be used as a boost dc/dc converterto charge the high-voltage battery 102 from the charging source.

When the MOSFETs 58 a-58 c are turned on, the current in the machineinductances increases, drawing energy from the low-voltage source andstoring it in the machine inductances. When the MOSFETs 58 a-58 c areturned off, some of this energy plus additional energy from thelow-voltage source is transferred to the high-voltage battery 102through the diodes 56 a-56 c.) The high-voltage battery 102 may becharged from a low-voltage source (for jump-starting purposes, forexample) using this method.

It should be recognized that this approach may also be utilized indual-voltage systems such as the system described below in conjunctionwith FIG. 15. In the case of a dual-voltage system, the low-voltagesource may be the low-voltage battery of the same vehicle, or it may besupplied from a different vehicle or source. Again, a means is providedfor selectively connecting the alternator machine neutral to the desiredlow-voltage source. In a dual-voltage system charging from its ownlow-voltage battery, this connection may be conveniently provided by arelay connecting the machine neutral to the positive terminal of thelow-voltage battery, for example.

A jump charging controller 105 couples a portion of the output voltageVo (or in some embodiments it may be desirable or preferable to couple aportion of the output current or both the output voltage and the outputcurrent) and provides a control signal to the control circuit 36. Thecircuit 105 regulates the output voltage Vo by changing the duty ratioof the switched mode rectifier circuit 54 to obtain a desired outputvoltage or current to charge the battery. It should be noted that oftenin the case where it is necessary to utilized this mode of operation,the associated engine and thus alternator will not be running and thusthe speed sensor 20 would provide a control signal corresponding to zeroalternator rpms.

It should be appreciated that although the alternator 25 is here shownas a field controlled alternator controllable via field currentregulator 26 and field controller 26 a, in those cases where speedsensor 20 can be omitted, the alternator can be provided as a so-calledpermanent magnet alternator. In this case, the output voltage iscontrolled via the SMR control circuit using known techniques.

Referring now to FIG. 15, a dual voltage system 106 which may be similarto the system 70 described above in conjunction with FIG. 12 includes aconnecting system 104, a first voltage source or battery 108 and asecond voltage source or battery 110. The connecting system 104 can beused to implement a method for charging the battery 110 at the output ofthe dual-rectified dual-controlled alternator system 106. The connectingsystem 104 may be provided as the type described above in conjunctionwith FIG. 14. In this case, the thyristors 78 a-78 c are not turned on,and the MOSFETs are used in conjunction with the machine inductance toallow charging of the high-voltage battery from the low-voltage chargingsource.

It will be now recognized by those of ordinary skill in the art thatthis same approach may be used with other switched-mode rectifierstructures, such as the alternative boost, Cuk, SEPIC, and push-pullrectifier structures described above in conjunction with FIGS. 6-10. Forthe rectifier systems described in FIGS. 6-10, the charging sourcevoltage may be either greater or less than the voltage of the batterybeing charged. Those of ordinary skill in the art will also recognizethat other machine connections (such as polyphase machines having morethan three phases) may also be used with the present approach. It isimportant for operation in accordance with the present invention thatthe machine inductances and switched-mode rectifier be used together asa switching power converter to charge the high-voltage battery from thecharging source.

Having described preferred embodiments of the invention, it will nowbecome apparent to one of ordinary skill in the art that otherembodiments incorporating their concepts may also be used. It is felttherefore that these embodiments should not be limited to disclosedembodiments but rather should be limited only by the spirit and scope ofthe appended claims. All publications and references cited herein areexpressly incorporated herein by reference in their entirety.

What is claimed is:
 1. A system for charging a first source having apositive and a negative terminal from a second source having a positiveand a negative terminal, the system comprising: an alternating current(ac) voltage source having an internal inductance; a switching powerconverter having a first set of terminals coupled to the ac voltagesource and a second set of terminals coupled to the positive andnegative terminals of the first source; a control circuit coupled tosaid switching power converter; a connecting system for selectivelyconnecting the positive terminal of the second source to the ac voltagesource; and a jump charging controller coupled to said control circuit,said jump charging controller operative to enable said switching powerconverter to be used in conjunction with the ac voltage sourceinductances as a dc/dc converter to charge the first source from thesecond source.
 2. The system of claim 1 wherein the positive terminal ofthe second source is coupled to the neutral point of the ac voltagesource.
 3. The system of claim 2 wherein the second source has a voltagelevel which is less than the voltage level of the first source.
 4. Thesystem of claim 3 wherein said ac voltage source is a wound-fieldalternator.
 5. The system of claim 3 wherein said ac voltage source is apermanent magnet alternator.
 6. The system of claim 1 wherein theswitching power converter is a switched-mode rectifier.
 7. A system forcharging a battery coupled to an output of a switching power convertercomprising: an ac machine coupled to an input of the switching powerconverter; a charging source having a positive terminal and a negativeterminal; and connecting means for selectively connecting the positiveterminal of the charging source to said ac machine.
 8. The system ofclaim 7 wherein said connecting means is coupled to a neutral point ofsaid ac machine.
 9. The system of claim 7 wherein the negative terminalsof the charging source and the battery are connected to system ground.10. The system of claim 7 wherein said charging source has a voltagelevel which is less than the voltage level of the battery.
 11. A methodfor charging a battery at an output of a switched-mode rectifiercomprising the steps of: (a) connecting a positive terminal of acharging source to an ac machine; (b) connecting a negative terminal ofthe charging source to system ground; (c) increasing a current in the acmachine to draw a first amount of energy from the charging source; (d)storing the first amount of energy in inductive elements of the acmachine; and (e) transferring the first amount of energy plus additionalenergy from the charging source to the battery.
 12. The method of claim11 wherein the step of connecting the positive terminal of the chargingsource to the ac machine includes the step of connecting the positiveterminal of the charging source to the neutral point of the ac machine.13. The method of claim 12 wherein the step of increasing the current inthe ac machine includes the step of providing a low impedance pathbetween output terminals of the ac machine to draw energy from thecharging source.
 14. The method of claim 13 wherein the step ofproviding a low impedance path between the output terminals of the acmachine includes the step of biasing switches in a switching powerconverter coupled to the ac machine into their conduction state.
 15. Themethod of claim 14 wherein the ac machine inductances in conjunctionwith the switching power converter cooperate to provide a dc/dcconverter to charge the battery from the charging source.
 16. The methodof claim 15 wherein the charging source has a voltage level which islower than the voltage level of the battery.
 17. The method of claim 16wherein the charging source corresponds to a low-voltage battery in avehicle in which the battery is disposed.
 18. The method of claim 16wherein the charging source corresponds to a low-voltage battery whichis not in a vehicle in which the battery is disposed.
 19. A system forcharging a battery coupled to an output of a switching power converterfrom a charging source having a positive terminal and a negativeterminal comprising: an ac machine coupled to an input of the switchingpower converter; and connecting means for selectively connecting thepositive terminal of the charging source to said ac machine, whereinsaid connecting means is coupled to a neutral point of said ac machine.